1. Field of the Invention
The present invention relates to a device for opening and closing shutter blade(s) of a shutter in a photographic camera.
2. Description of the Prior Art
It has been proposed in a multi-focus camera or a zoom lens camera to incorporate therein an electromagnetic actuator of the rotary magnet type consisting of a rotatable permanent magnet rotor operatively coupled with the shutter blade(s) and a stationary coil surrounding the rotor and electromagnetically cooperating therewith. The rotor is adapted to be rotated in one direction when a current is applied to the stationary coil in one direction, while the rotor is rotated in the opposite direction when a current is applied to the stationary coil in the opposite direction, so that the shutter blade(s) can be opened and closed depending upon the direction of the current applied to the stationary coil thereby permitting the exposure of the camera to be achieved.
In such a shutter of the prior art, however, it suffers from various problems in actuating the shutter blade(s).
(1) In case the opening of the shutter blade(s) is effected by mechanical means such as a spring, the performance or the opening speed of the shutter blade(s) can be varied by merely adjusting the mechanical property of the mechanical means, the force of the spring, for example, so as to achieve the desired performance of the individual shutter.
However, in the case of a shutter in which an electromagnetic actuator such as a stepping motor is used in opening the shutter blade(s), a complicated circuit is required for adjusting the opening speed of the shutter blade(s) so as to vary the duty factor of the driving pulses of the stepping motor used as the actuator, thereby rendering the shutter to be complicated in construction and the cost of the shutter to be expensive. In case a D.C. motor is used as the power sourse, an encoder or a sensor is required for effecting the feedback operation which renders also the device to be complicated and expensive.
(2) In an electromagnetic actuator, since the torque for rotating the actuator is generated by supplying the current thereto, the current must be applied to the actuator at all times during the actuation of the shutter blade(s) from the start of opening the shutter blade(s) and maintaining the same at the opened position until the shutter closing current is supplied to the actuator so as to close the shutter blade(s) after the exposure. Therefore, the shutter blade(s) tend to seriously bounce when the same strike against stopper means at the end of the opening operation, because the shutter opening current countinues to be supplied to the actuator. Therefore, the exact exposure can not be expected.
(3) In a camera incorporating therein an electromagnetic actuator described above usually includes a spring urging the shutter blade(s) toward the closed position so that the shutter blade(s) can be held closed stably without applying the shutter closing current to the actuator. Alternatively, a magnetic body such as an iron pin is provided at an appropriate position adjacent to the permanent magnet rotor so that it cooperates with the magnetic flux of the rotor so as to urge the rotor in the shutter closing position thereby maintaining the shutter blade(s) stably in the closed position.
However, in a shutter adapted to be incorporated in a single lens reflex camera of the lens shutter type, the shutter must be held opened so as to be able to introduce the light from the object to be photographed through the opened shutter and to view the object by preventing the light from the object from being incident to the photosensitive material by actuating a light intercepting member or a reflexing plate before the exposure. After the range finding operation, the shutter is once closed while the light intercepting member is moved out of the optical axis toward the photosensitive material and then the shutter blade(s) are opened and closed for the exposure.
In such a shutter, the shutter blade(s) must be stably held at the fully opened position by means such as a spring when the electric source is intercepted. Therefore, the shutter closing speed is necessarily lowered making it imposible to effect an exposure of very short shutter time, because the shutter blade(s) must be closed by the shutter closing current against the action of the spring.